Effect of Vertical, Horizontal, and Combined Plyometric Training on Explosive, Balance, and Endurance Performance of Young Soccer Players

Abstract

Ramírez-Campillo, R, Gallardo, F, Henriquez-Olguín, C, Meylan, CMP, Martínez, C, Álvarez, C, Caniuqueo, A, Cadore, EL, and Izquierdo, M. Effect of vertical, horizontal, and combined plyometric training on explosive, balance, and endurance performance of young soccer players. J Strength Cond Res 29(7): 1784–1795, 2015—The aim of this study was to compare the effects of 6 weeks of vertical, horizontal, or combined vertical and horizontal plyometric training on muscle explosive, endurance, and balance performance. Forty young soccer players aged between 10 and 14 years were randomly divided into control (CG; n = 10), vertical plyometric group (VG; n = 10), horizontal plyometric group (HG; n = 10), and combined vertical and horizontal plyometric group (VHG; n = 10). Players performance in the vertical and horizontal countermovement jump with arms, 5 multiple bounds test (MB5), 20-cm drop jump reactive strength index (RSI20), maximal kicking velocity (MKV), sprint, change of direction speed (CODS), Yo-Yo intermittent recovery level 1 test (Yo-Yo IR1), and balance was measured. No significant or meaningful changes in the CG, apart from small change in the Yo-Yo IR1, were observed while all training programs resulted in meaningful changes in explosive, endurance, and balance performance. However, only VHG showed a statistically significant (p ≤ 0.05) increase in all performance test and most meaningful training effect difference with the CG across tests. Although no significant differences in performance changes were observed between experimental groups, the VHG program was more effective compared with VG (i.e., jumps, MKV, sprint, CODS, and balance performance) and HG (i.e., sprint, CODS, and balance performance) to small effect. The study demonstrated that vertical, horizontal, and combined vertical and horizontal jumps induced meaningful improvement in explosive actions, balance, and intermittent endurance capacity. However, combining vertical and horizontal drills seems more advantageous to induce greater performance improvements.

Introduction

Sprinting, jumping, and change of direction speed (CODS) are important determinants for success in adult (⁴²) and young soccer players (⁴). Although sprinting only contributes up to 3% of the total distance covered in children's games (⁴), most crucial moments such as winning ball possession, scoring, or conceding goals depend on it (⁴²). It has been proposed that the high degree of plasticity in neuromuscular development during preadolescence, combined with appropriately timed implementation and progression of integrative neuromuscular training (e.g., supplemental training combining general and specific strength and conditioning exercises, such as plyometrics), may allow for strengthened physical development that contributes favorably to athleticism into adulthood (³⁵). Selection of relevant training methods, which contribute to the development of soccer-related explosive activities from a young age, must therefore be on the forefront of the practitioner's mind to increase the chance of the player's success (²¹). As demonstrated by others (²⁷), power increases with age and is related to the improvement of 1 or both of its components, velocity or force. Velocity at peak power appeared to be the changing factor in peak power output before peak height velocity (PHV—a somatic biological maturity indicator that reflects the maximum velocity in statural growth during adolescence), whereas force at peak power was a more important determinant during and post-PHV (²⁷). Based on these findings, training for velocity in the initial phase of a player development has been recommended (³⁰).

Unloaded or body mass plyometric training provides such a high velocity training stimuli in young soccer players (^10,31–32), affecting maximal muscle power of different movements (³⁰). The effectiveness of plyometric training depend on several factors such as eccentric overloading, segmental coordination, specificity of joint angle, and angular velocities (^13,45). However, few studies offer optimum plyometric training design in relation to exercise selection and their associated direction of force production (⁹). Vertical and horizontal explosive exercises (e.g., jumping) may depend on different neuromuscular capabilities. For example, the stretch-shortening cycle (SSC) contributes less to horizontal than vertical jump performance, because a vertical load on the musculotendinous unit generates a bigger stretching load, allowing greater accumulation and use of elastic energy during the concentric phase (^18,28). As specificity is an important requirement for training induced adaptations, performance changes in explosive neuromuscular actions may require specific training strategies. Although it is common to find muscle power evaluations incorporating bilateral, vertical and acyclic muscle actions (e.g., squat jump, countermovement jump) in the literature, unilateral, horizontal and cyclic muscle actions may best reflect competitive athletic performance changes (²⁸). Most movements implicate a combination of vertical, horizontal, and lateral force production, especially where speed and CODS are important in a multidirectional sport such as soccer (^18,31). Recent reviews on the topic (^41,43) suggest that plyometric training should include horizontal and vertical movements to enhance vertical and horizontal power. Most previous studies have prescribed vertical plyometric exercises in young soccer players (^14,49–50) or have been vertical dominant (^10,32). Only 1 plyometric intervention provided an equal vertical and horizontal stimulus (³¹). However, no studies have established the different effect of vertical, horizontal, or the combination of both types of plyometric exercises on explosive performance of young soccer players.

Apart from sprinting, jumping, and CODS, explosive training may also enhance kicking and endurance and balance performance. Although explosive training has consistently showed a positive effect on kicking performance (^12,24), its effect on endurance remains controversial. Studies in youth soccer players did not demonstrate improvement in V[Combining Dot Above]O₂max (³²) or lactate thresholds (¹⁴), whereas others (⁵⁰) demonstrated an increase in performance during a Yo-Yo intermittent recovery level 1 test (Yo-Yo IR1). Also, balance may be a fundamental quality for both execution of technical movements and prevention of injuries (⁴³); however, the plyometric training effect on young soccer players balance is currently unknown (^5,36).

Given the limitations previously cited, our objective was to assess the effect of plyometric exercises in the vertical plane, horizontal plane, and their combination on several explosive, balance, and endurance performance measures in young soccer players.

Methods

Experimental Approach to the Problem

This study was designed to address the question of how a short-term plyometric training program of moderate frequency (2 sessions per week), incorporating vertical, horizontal, or vertical and horizontal exercises would affect differently jumping, sprinting, kicking, endurance, CODS, and balance performance measures in young soccer players. After the initial measurements, participants were randomly assigned to a control group (CG, n = 10), or 1 of the 3 training group: vertical plyometric training group (VG, n = 10), horizontal plyometric training group (HG, n = 10), and combined vertical and horizontal plyometric training group (VHG, n = 10). Measurements were repeated postintervention.

Subjects

Forty young male soccer players (aged between 10 and 14 years) with no background in regular strength or plyometric training volunteered to participate in this study. Exclusion criteria included participants with (a) potential medical problems or a history of ankle, knee, or back pathology that compromised their participation or performance in the study and (b) any lower extremity reconstructive surgery in the past 2 years or unresolved musculoskeletal disorders. Despite not pair-matching individuals based on an independent variable, there were no significant differences between groups' characteristics at baseline (Table 1).

Table 1: Descriptive data of the CG, VG, HG, and VHG.^*

Participants (and their parents or guardians) were informed about the experimental procedures and about possible risks and benefits associated with participation in the study and signed an informed consent before any of the tests were performed. The study was conducted in accordance with the Declaration of Helsinki and was approved by the institutional review board for use of human subjects from the university.

Testing Procedures

Participants were familiarized with the test procedures 2 weeks before the initial assessment to reduce learning effects. Measurements were undertaken 1 week before and after the intervention. To reduce the potential effect of cumulative fatigue on dependent variable outcomes, before and after intervention, athletes had 7 days of rest between the last training session and first measurement session. In addition, in the week preceding performance measurements, no competitive game was performed, and athletes were asked to reduce the intensity of their regular physical education classes. Tests were completed in 3 days and always administered in the same order, at the same time of the day, by the same investigators, with >48 hours of rest from physical activity. Participants were instructed to use the same athletic shoes and clothes during all testing sessions. Tests were conducted indoors on a wooden surface. Throughout testing, an investigator subject ratio of 1:1 was maintained. Ten minutes of standard warm-up (i.e., submaximal running with CODS, 20 vertical and 10 horizontal submaximal jumps) were executed before testing.

On day 1, the standing height, sitting height, body mass, body composition, vertical (VCMJ) and horizontal (HCMJ) countermovement jump with arms for maximal vertical and horizontal distance, 20-cm drop jump reactive strength index (RSI20), and the multiple 5 bounds test (MB5) for maximal horizontal distance were completed. Anthropometric measurements were taken using a stadiometer (Bodymeter 206; SECA, Hamburg, Germany) and an electrical scale (BF100_Body Complete; Beurer, Ulm, Germany), and the athletes maturity status was determined using predicted years from age of PHV (i.e., PHV offset) (³³). Jump height during the VCMJ (i.e., centimeter) and jump height divided by contact time during the RSI20 (i.e., mm·ms⁻¹) were measured with an electronic contact mat system (Ergojump; Globus Italy, Codogne, Italy). Players could use their arms during the VCMJ, while keeping their arms akimbo during the RSI20. Takeoff and landing was standardized to full knee and ankle extension on the same spot. The participants were instructed to maximize jump height and minimize ground contact time during the RSI20 after dropping down from a 20-cm drop box. HCMJ and 5 multiple bounds test (MB5) were completed with arm swings and measured with a tape measure to the nearest centimeter. All jump protocols have been previously described (⁴⁰).

On day 2, the 15-m sprint time for acceleration and 30-m sprint time for maximal speed, the maximal kicking velocity test (MKV), and a soccer-specific CODS test (³¹) were performed. The 15 and 30-m sprint time was measured using single beam infrared photoelectric cells (Globus Italy) placed at 0, 15, and 30 m and leveled ∼0.7 m above the floor (i.e., hip level) to capture the trunk movement rather than a false trigger from a limb. The starting position was standardized to a still split standing position with the toe of the preferred foot forward and behind the starting line. Sprint start was given by a random sound, which triggers timing. The CODS test has been described elsewhere (³¹), and the timing system and procedures were the same as the 30-m sprint. For the MKV, participants kicked a size 5 soccer ball (Nike Seitiro, FIFA certified) for maximal velocity measured by a radar gun (Sports Radar Speed Gun SR3600; Sports Radar, Homosassa, FL), according to a previously described protocol (²). Basically, participants performed a maximal instep kick with their dominant leg after a run up of 2 strides, directed toward a goal net with a cue to aim (i.e., a vertical square target placed in its center) to increase the reliability of the test. The distance between the ball and the target was 4 m. Participants were given 2 practice and 3 valid maximal trials, with ≥1 minute of rest between trials.

On day 3, bilateral balance and the Yo-Yo IR1 were completed. As previously reported for balance testing (¹), participants completed 2 stability tests performed on a balance platform at a sample rate of 1,000 Hz (Bertec BP5050 balance plate platform; Bertec, Corp., Columbus, OH, USA): (a) normal stance, eyes open and (b) normal stance, eyes closed. The average of 2 trials for each test was used for subsequent analysis. Both anterior-posterior and medial-lateral data were collected during each trial. The Yo-Yo IR1 test was executed as previously described (¹⁹). Before testing, participants perform a warm-up consisting of the first 4 running bouts in the test. Participants achieved a mean of 206 b·min⁻¹ (i.e., 98% of theoretical maximal heart rate [220-age]) at the end of the test, suggesting maximal effort.

Training Design

The current experiment was completed during competition period. Participants performed plyometric drills as a substitute for some soccer drills within the usual 90-minute practice twice per week for 6 weeks. Before beginning the training period, participants were instructed to properly execute all the exercises to be performed during this period. In addition, all training sessions were supervised using a trainer to player ratio of 1:4, and particular attention was paid to demonstration and execution. The plyometric drills were performed just after the warm-up and separated with a minimum of 48 hours (including games). All groups completed the same amount of total jumps, using the same surface and time of the day for training, with the same rest intervals between jumps (i.e., 15 seconds for acyclic jumps) and series (i.e., 60 seconds). Aside from the formal training intervention, all participants attended to their regular physical education classes.

Vertical plyometric training group and HG executed horizontal and vertical exercises, respectively, whereas VHG combined them. All groups use arm swing during jumps, combining cyclic and acyclic, in addition to unilateral and bilateral jumps. Participants were asked to achieve maximal vertical height and horizontal distance for acyclic jumps and with minimum ground contact time for cyclic jumps. Maximal intensity during training was verified in a randomly assigned subsample of participants (2 from each plyometric group; n = 6) during 2 randomly assigned training sessions, by measuring contact times, height, and distance of jumps drills, using the same procedures as during testing. A detailed description of the training program is depicted in Table 2.

Table 2: Six-week plyometric training program.^*†

To control that all soccer players receive the training load during intervention, the session rating of perceived exertion (RPE) was determined as previously described (¹⁷) and is reported in Table 1. Briefly, each athlete's session RPE was collected about 30 minutes after each soccer training session and game to ensure that the perceived effort was referred to the whole session rather than the most recent exercise intensity. In this study, the Chilean translation of the 10-point category ratio scale (CR10-scale) modified by Foster et al. (¹¹) was used. This scale was modified to better reflect the Chilean idiomatic English.

Statistical Analyses

All values are reported as mean ± SD. Relative changes (%) in performance and standardized effects (SE) are expressed with 90% confidence limits. Normality and homoscedasticity assumptions were checked, respectively, with Shapiro-Wilk and Levene tests. To determine the effect of the intervention on performance adaptations, a 2-way analysis of variance with repeated measurements (4 groups × 2 times) was applied. When a significant F value was achieved across time or between groups, Tukey post hoc procedures were performed to locate the pairwise differences between the mean values. The α level was set at p ≤ 0.05 for statistical significance. All statistical calculations were performed using STATISTICA statistical package (version 8.0; StatSoft, Inc., Tulsa, OK, USA). In addition to this null hypothesis testing, data were also assessed using magnitude of based inference statistics (¹⁶). Threshold values for assessing magnitudes of SE (changes as a fraction or multiple of baseline SD) were 0.20, 0.60, 1.2, and 2.0 for small, moderate, large, and very large, respectively (¹⁶). Magnitudes of differences in training effects between groups were evaluated nonclinically (¹⁶): if the confidence interval overlapped thresholds for substantial positive and negative values, the effect was deemed unclear. The effect was otherwise clear and reported as the magnitude of the observed value with a qualitative probability, as above. We obtained a relatively high intraclass correlation coefficient and low coefficient of variation for the vertical countermovement jump with arms test (0.91 and 3.5%, respectively), horizontal countermovement jump with arms test (0.95 and 3.7%, respectively), 20-cm drop jump RSI (0.94 and 3.7%, respectively), multiple 5 bounds test (0.95 and 4.8%, respectively), MKV test (≥0.91 and <4.4%, respectively), 15-m sprint time test (0.93 and 2.5%, respectively), 30-m sprint time test (0.95 and 1.9%, respectively), CODS test (0.91 and 3.5%, respectively), anterior-posterior normal stance eyes open test (0.94 and 4.1%, respectively), medial-lateral normal stance eyes open test (0.94 and 4.1%, respectively), anterior-posterior normal stance eyes closed test (0.81 and 6.9%, respectively), and medial-lateral normal stance eyes closed test (0.94 and 4.3%, respectively).

Results

Before training, no significant differences were observed between groups in vertical CMJ, horizontal CMJ, RSI, multiple bound test (Table 3), kicking velocity, 15-m and 30-m sprint time, CODS, Yo-Yo IR1 (Table 4), or balance (Table 5) test performance.

Table 3: Training effects (with 90% confidence limits) for the jump performance variables for the CG (n = 10), VG (n = 10), HG (n = 10), and VHG (n = 10).^*

Table 4: Training effects (with 90% confidence limits) for the soccer-specific explosive and endurance performance variables for the CG (n = 10), VG (n = 10), HG (n = 10), and VHG (n = 10).^*

Table 5: Training effects (with 90% confidence limits) for the balance performance variables for the CG (n = 10), VG (n = 10), HG (n = 10), and VHG (n = 10).^*

No statistically significant changes in the CG were observed, although a small meaningful change (0.23 SE) in Yo-Yo IR1 was noted (Table 4). In comparison with the CG, horizontal training groups (i.e., HG and VHG) showed a significantly (p ≤ 0.05) higher performance change and SE in horizontal CMJ and multiple bound test, whereas vertical training groups (i.e., VG and VHG) showed a significantly (p ≤ 0.05) higher performance change and SE in the RSI (Table 3). Also, in comparison with the CG, the VHG showed a significantly (p ≤ 0.05) higher performance change and SE in kicking velocity, 15-m sprint time, 30-m sprint time, CODS, Yo-Yo IR1 (Table 4), and balance (both, medial-lateral and anterior-posterior) test (Table 5).

Except for the HG and VG groups in the vertical CMJ and multiple bound test (respectively), all plyometric training groups showed a significant (p ≤ 0.05) increase and small-to-moderate meaningful SE in vertical CMJ, horizontal CMJ, RSI, and multiple bound test (Table 3). No statistically significant differences in performance changes were observed between training groups (Table 3), although vertical training was more effective to a small effect at improving vertical CMJ performance in comparison with horizontal training. In addition, horizontal training (i.e., HG and VHG) was more effective to a small effect than vertical training (i.e., VG) at improving horizontal CMJ and multiple bound test performance (Table 6).

Table 6: Differences between CG (n = 10), VG (n = 10), HG (n = 10), and VHG (n = 10) in the training effects (with 90% confidence limits) on performance variables.^*†

Although the 3 plyometric training groups had a small to moderate meaningful SE in kicking velocity, 15-m and 30-m sprint, CODS, Yo-Yo IR1 (Table 4), and balance test (Table 5), only the combined vertical and horizontal program had a statistically significant (p ≤ 0.05) effect in all performance test. In addition, the combined vertical and horizontal training group improved more (small effect) than the VG and HG in 30-m sprint, CODS, and balance performance test and also improved more (small effect) than the VG in kicking velocity and 15-m sprint performance test (Table 6). The HG improved more (small effect) than the VG in 15-m sprint performance test to a small effect (Table 6).

Discussion

The results of this study indicated a specificity of training effect, where the use of vertical exercises induced a significantly greater increase in performance tests in the vertical plane, whereas the use of horizontal exercises induced a significantly greater increase in performance tests in the horizontal plane. Also, the results indicate that, compared with CG, only a combination of vertical and horizontal training stimulus achieved a significantly greater increase in almost all (i.e., 9 of 13) performance measures. Finally, our results demonstrated that the combination of soccer drills and specific explosive training with no additional training time in-season optimizes general and soccer-specific explosiveness, balance, and endurance performance in young soccer players.

The results from the VCMJ, HCMJ, RSI20, and MB5 test all demonstrated the training principle of specificity in plyometric training (^25,41), which can be explained by factors such as eccentric overloading, segmental coordination, and muscular activation (⁴⁵). Groups trained in the specific direction of the jump test consistently improved to a greater extend (Table 6) and increased the gap with the CG (Table 3). The magnitude change in VCMJ was similar or even higher than previously reported for similar slow SSC muscle actions (SE = 0.51–0.75) (^10,31,50) after explosive training with young soccer players using an intervention of similar duration or number of sessions. The magnitude change in RSI20 was similar than previously reported (SE = 0.41–0.90) (³⁹) after plyometric training with young soccer players. Considering the necessity to produce a high rate of force development in explosive actions (²⁹), the improvement in RSI may have enhanced physical parameters of game performance. The improvement observed could have been induced by various neuromuscular adaptations (²³); however, because no physiological measurements were made, only speculations are possibly. Finally, the magnitude change in MB5 in this study was similar than previously reported (SE = 0.62–0.63) (^10,31) after explosive training with young soccer players. An increase in MB5 may be achieved by motor coordination adaptations, which can be related to the specificity of movements used during training (¹⁰). The fact that the HG and VHG performed horizontal plyometric drills, achieving a significantly higher performance change in MB5 than the CG, would support such contention considering the horizontal nature of the MB5.

Although previously it has been shown that a combination of vertical and horizontal (⁴⁶) or unilateral and bilateral (¹²) plyometric exercises can increase MKV, this is the first study to compare the effects of vertical, horizontal, and combined vertical and horizontal jumps, including unilateral and bilateral drills, in MKV of young soccer players. All training programs induced a meaningful increase in MKV performance, but the VHG improved more to a small effect and significantly in MKV compared with VG and CG (Tables 4 and 6), suggesting that a combination of vertical and horizontal exercises may induce higher MKV performance changes in young soccer players than plyometric drills applied in only 1 plane of direction. Although differences in type of training program applied make comparisons between different studies difficult, others have found significant increases in kicking performance after plyometric training in young soccer players (^12,24,32), in both dominant and nondominant kicking legs (¹²). It has been suggested that the increased MKV performance may be attributed to increased strength and power of legs' extensor muscles (³²), agonists-antagonists muscle coordination, and greater recruitment of motor units (¹²). It may be that these neuromuscular and strength-power adaptations had an effect on the biomechanical factors related to kicking performance, such as maximum linear velocity of the toe, ankle, knee, and hip at ball contact (²⁰), which may have cumulatively or individually contributed to a higher ball kicking velocity.

All training groups showed a meaningful decrease (i.e., SE between 0.30 and 0.99) in 15 and 30-m sprint times, but only the VHG demonstrated a significant (p ≤ 0.05) change compared with CG (Table 4). The results also demonstrated a small beneficial effect of horizontal stimulus for 15-m sprint time and implementing some vertical stimulus to decrease 30-m sprint time (Table 6). Although others have found similar effects (SE = 0.29–1.1) in sprint performance after plyometric training in young soccer players (^31–32), this was the first study to compare the effects of vertical, horizontal, and combined vertical and horizontal plyometric training in 15 and 30-m sprint times in young soccer players. As the training intervention in the HG and VHG incorporates horizontal stimulus, this may had increased the chances to gain adaptations considering the importance of horizontal force production and application in sprint performance (^18,34). This agrees with previous studies, where vertical plyometric training fails to improve sprint performance in young soccer players (³⁹). The higher increase in sprint performance in VHG coincides with previous results (⁴⁸). The CG did not exhibit a meaningful or statistically significant improvement in sprint performance in this study, and conducting soccer training only in-season may even induce decay in sprint performance (³⁹). These observations reinforce the value of an independent explosive training program to enhance acceleration and maximal sprint ability of young soccer players during their in-season. The increase in both acceleration and maximal running velocity can be related to leg power (⁷), which can be increased with plyometric training in young soccer players (^6,38). Although acceleration improvement may be more related with the slower SSC and rate of power production nature of the acyclic jumps (i.e., countermovement jumps) (⁸) performed during training, leg stiffness developed through stiffer plyometric exercises (i.e., cyclic jumps) may transfer to maximal running velocity improvement (⁷). Therefore, in addition to the more favorable sprint performance changes observed after the combination of vertical and horizontal plyometric exercises in this study, these results reinforce the need to add variation in plyometric cycle (i.e., cyclic and acyclic movement) to improve the different neuromuscular variables related with sprint performance.

This study demonstrated that all plyometric training stimulus induced a meaningful increase in CODS performance, with a small to moderate SE (Table 4). However, only VHG showed a statistically significant increase and significant change in comparison with the CG (Table 4). Also, the VHG program was more effective at improving CODS performance compared with VG and HG to a small effect (Table 6). All training programs contained exercises designed to induce short contact times and subsequently increase RSI, which may predict the ability to change directions while running (⁵¹). Also, an improved CODS performance may be related to changes in power development or increased eccentric strength level, which can impact COD performance during the deceleration phase (⁴⁷). This study demonstrated that combined horizontal and vertical plyometric training was more efficient at driving multiple explosive-related adaptations, which seemed to have transferred to CODS performance in young soccer players. The important differences between CODS tests used among studies make results comparison difficult; however, using the same CODS test as in our study, others have found an even greater change in performance after plyometric training in young soccer players (9.7%; SE = 2.8) (³¹). Because greater change occurred in the multiplane training stimulus group in this study, it can be argued that greater magnitude of change in the study of Meylan and Malatesta (³¹) may be because of more variety in stimulus but also the intervention duration in comparison with this study.

Our results demonstrated a significant and small meaningful increase in Yo-Yo IR1 performance in all training groups (Table 4). In young soccer players, plyometric training may not induce a significant increase in underlying aerobic qualities such as V[Combining Dot Above]O₂max (³²) or lactate threshold (¹⁴) but may still have a meaningful effect on a intermittent recovery endurance performance test with repeated changes of direction (⁵⁰). This discrepancy is likely related to the fact that the change in explosive performance after a plyometric training can contribute to the change of direction during an intermittent test (e.g., Yo-Yo IR1 or 30–15 intermittent fitness test) with change of direction (³) or running economy (²⁶), independently from the influence on V[Combining Dot Above]O₂max (³²) or lactate threshold (¹⁴). As previously stated, all training groups demonstrated a meaningful change in reactive strength (i.e., RSI20), which may transfer into improved running economy and enhance aerobic performance independently of others aerobic indicators (e.g., V[Combining Dot Above]O₂max or lactate threshold) (³⁷).

A novel aspect of this study was to analyze the effect of plyometric training on youth soccer players balance and the different effect of various plyometric training interventions on balance capability. Our results showed that although all training groups achieve a meaningful change in all measures of anterior-posterior and medial-lateral balance (Table 5), only VHG achieved a significantly higher performance change in anterior-posterior balance in comparison with CG (Table 5) and also a higher SE to a small effect in both medial-lateral and anterior-posterior balance compared with VG, HG, and CG (Table 6). Improvements in balance after plyometric training have already been shown after 6 weeks (^36,52) and in young athletes (^5,36). Because balance improvements may not only result in an increased athletic performance but also in reduced lower-extremity injury risk in soccer players (⁵²), our results reinforce the value of plyometric training as an effective strategy to reduce injury risk in young athletes. There was no significant difference in medial-lateral balance postintervention in all the groups (Table 5). As the training was anterior-posterior in nature for all groups, which resulted in better balance in that plane, there is an argument to also include medial-lateral plyometric-based exercises to improve the associated balance capability. The improvement in balance performance may be related to improved cocontraction of lower-extremity muscles (³³) or changes in proprioception and neuromuscular control (¹⁵), which appeared to be direction specific based on this study. Furthermore, to enhance functional balance, initial emphasis on landing mechanics (i.e., acyclic jumps) during plyometric training may prove more beneficial for complex high speed unilateral repetitive dynamic tasks taxing postural control (i.e., sprinting, change of direction) as compared with fast SSC plyometrics (i.e., cyclic) (⁵).

Practical Applications

The replacement of some soccer drills with high-intensity plyometric exercises may positively affect jump, sprint, kicking, CODS, endurance, and balance performance in young soccer players during the in-season period. These adaptations can be achieved in the short-term and may potentially increase competitive performance and may reduce injury risk. When programming, the practitioner must be cognizant of the training response being specific to the direction of force production in the plyometric drills. The combination of vertical and horizontal jump stimulus was more advantageous to youth soccer players to gain meaningful improvements in explosive performance, balance, and intermittent aerobic capacity than vertical or horizontal plyometric stimulus alone. Such approach appeared relevant to the multidirectional nature of soccer but may not have the same positive effect where a direction of force production is dominant (e.g., volleyball). As most sports are unilateral in nature, it is recommended to find the right balance and progression between bilateral and unilateral training stimulus, although whether a training stimulus is more advantageous than the other is still to debate. A combination of acyclic to cyclic jump may also prove relevant to the specificity of the sport and the level of athlete movement competency and dynamic balance. Finally, although plyometric training can induce an increase in explosive, endurance, and balance performance in young soccer players, to optimize training adaptations, this training strategy should be adequately applied in a more complex training plan that incorporates other explosive (e.g., sprints), endurance, technical, and tactical-oriented training methods.

Acknowledgments

The authors disclose professional relationships with companies or manufacturers who will benefit from the results of this study. The results of this study do not constitute endorsement of the product by the authors of the National Strength and Conditioning Association.